We have developed a digital computer model for the simulation of the body surface electrocardiogram (ECG) in order to study the information it contains about regional myocardial ischemia. The electrical activity of the ventricles is represented by 23 dipoles with time varying moments located in a homogeneous volume conductor with the shape of an adult male torso. Excitation sequence and cellular action potential data taken from the literature are used to determine the dipole moments at each instant of time throughout simulated cardiac cycles (ventricular activation and repolarization) for normal and ischemic conditions. Emphasis in the simulation studies will be placed on defining quantitative relationships that may exist between the surface ECG and the site and size of the ischemic injury. An understanding of such relationships and their ambiguities would add to the understanding of the limits on the applicability of such diagnostic procedures as precordial S-T segment mapping. It would also be useful in suggesting new procedures which could provide more quantitative information about ischemic injury than is available using current electrocardiographic techniques. Simulated electrocardiograms and body surface maps will be compared with ECG's recorded at multiple sites on patients with acute ischemia or infarction.